1. FIELD OF THE INVENTION
The present invention relates to an optical communication line or cable having optical fiber couplers for dividing light signals into a plurality of branches at a desirable split ratio used in an optical communication system or optical sensor system accommodating an optical fiber for example.
2. DESCRIPTION OF THE PRIOR ART
As an important component of an optical communication system or optical data link, there is provided an optical turnout or branch connection for dividing light signals transmitted from a light source to a plurality of optical fibers at a desired split ratio. There is used an optical fiber coupler in an optical communication system for coupling optical fibers as a kind of an optical branch connection. Such an optical fiber coupler is made in such a manner that, after a plurality of optical fibers are fused by melting in twist or in parallel, the fused optical fibers are heated to be melted by means of a heater such as an acetylene burner so that the melted optical fibers are extended under applying a constant tension, thereby obtaining an optical fiber coupler.
As a type of products of such an optical fiber coupler, there are two types such as so called a "2.times.2 type coupler" which is made of two optical fibers (see e.g., "Low-Loss access couple for multimode optical fiber division networks" reported by B.S. Kawasaki et al. in APPLIED OPTICS, Vol. 16, No. 7, in 1977 (page 1794 to 1795)) and so called a star type coupler which is made by coupling three or more fibers (see, e.g., "Quartz Taper Fiber Star Coupler" reported by Oshima in the Electronic Communication Society All Japan General Meeting No. 2299 in 1982).
In case the 2.times.2 type optical couplers are used as essential components of an optical communication subscriber's network, it is possible to divide a tree shaped optical communication network comprising optical fiber couplers 1 and subscribers 2 as shown in FIG. 1. In case the star couplers are used as essential components of a subscribers' network, it is possible to divide a star shaped optical communication network comprising optical fiber couplers 1 and subscribers 2 as shown in FIG. 2.
FIG. 3 shows a detailed structure of the optical fiber coupler 1 in the optical communication subscriber's network shown in FIG. 2, wherein the star coupler of 1.times.4 type is made in such a manner that, after graded index multi-mode fibers 1L, 2L, 3L and 4L are twisted together, the twisted fibers are heated to be melted and extended under applying a constant tension, whereby the star coupler of 1.times.4 type is made, having a taper portion 5 with its diameter reduced due to the extension thereof. When light signals are injected through an optical fiber 1F for example and guided to the taper portion 5, the input light signals are transferred and divided to the optical fibers 1L, 2L, 3L and 4L which are twisted and fused together.
In order to operate the subscriber's network having such an optical fiber coupler formed as described above in the same manner as is designed, it is necessary that the optical fiber coupler should satisfy the desired characteristics and, for example, in order to equally divide the input light signals to a plurality of output signals of the optical fiber coupler 1, the irregularity of the output power at each output port of the optical fiber coupler 1 must be reduced to be as small as possible. Moreover, the optical fiber coupler is so designed that the predetermined desired characteristics can be obtained when the optical fiber coupler is operated under a steady-state excitation of input light signals emitted by such as a light emitting diode (referred to as LED hereinafter), and then the input light power of the optical fiber coupler is equally divided to the optical fibers 1L, 2L, 3L and 4L twisted together in the taper portion 5 with power distribution as shown by a real line for example including higher modes of the output light signals being excited as shown by a real line in FIG. 3.
While, nowadays, it becomes possible to get a semiconductor laser (referred to as LD hereinafter) with high output power at a low cost equivalent to that of a LED, upon using a LD as a light source in an optical communication system, it is expected that the transmission distance can be extended and that the number of subscribers can be increased because of the high output power of LD.
However, since LD has an intense directivity of light projection with its main lobe projected, when the LD is coupled with the multi-mode fibers, only dominant lower modes of the input light power are excited in the optical fiber 1L for example as shown by dotted line in FIG. 3, and therefore, sufficient light power can not be divided to the other optical fibers 2L, 3L and 4L in the taper portion 5 of the optical fiber coupler, resulting in that there is a problem, being that there occurs irregularity of the output power among the respective output ports so that the desired characteristics of the optical fiber coupler can not be obtained when a LD is used as a light source.